When structurally delicate agents, such as proteins, peptides, gene materials, viruses, vaccines, antibodies or liposomes are loaded in degradable polymer-based drug delivery systems, the contact with organic solvents (which are required to dissolve the polymers) often causes denaturation of these agents [1-3]. To avoid this denaturation, proteins and peptides were prepared into solid particles with sugars or polysaccharides prior to encapsulation into water-insoluble polymer devices [4-7]. For sustained-release purposes, the protein-containing sugar or polysaccharide particles should be small enough (<10 μm) to avoid severe burst in the initial stages of release [4, 5]. Pre-loading proteins or peptides into small particles of aqueous polymers prior to encapsulation by degradable polymer devices may also improve release kinetics of the loaded devices by minimizing burst and incomplete release [5, 8].
Loading proteins into fine polysaccharide particles can be a formidable task due to the fact that reported particle-forming methods generally involve the use of organic solvents, interfacial tension (such as water-oil interfaces or water-air interfaces and other hydrophilic/hydrophobic interfaces), shear stress, elevated temperatures, and other conditions that are hazardous to the loaded proteins. However, there have been some progresses in the field.
We (Jin, et al.) disclosed a unique system, aqueous-aqueous emulsion, for loading delicate proteins into polysaccharide fine particles under conditions without organic solvents, strong interface tension, strong shear stress, and elevated temperatures [5, 8]. Proteins can be loaded into the dispersed polysaccharide phase thermodynamically by preferential partition under such mild conditions.
This system, however, is compromised with some drawbacks. For example, it requires relatively concentrated aqueous solutions of two hydrophilic polymers (a polysaccharide and a PEG solution) and usage of a third water-soluble polymer, a polyelectrolyte with a negative charge on its backbone, to form an aqueous-aqueous emulsion and to stabilize the dispersed polysaccharide phase from fusion [5, 8]. If solubility of the protein is low, the ratio of polysaccharide/protein will be too high so that loading capacity for proteins will be too low (since the concentration of polysaccharide must be high enough for phase separation). In addition, some proteins may interact strongly with the polyelectrolyte, which is required for forming a stable aqueous-aqueous emulsion, to form ionically complexed aggregates.
Consequently, there is much demand for a method of encapsulating structurally delicate proteins and/or peptides into a matrix of degradable polymer-based drug delivery systems, where the proteins and/or peptides will not be denatured in the process and will be released in a controlled manner. Pre-loading proteins into polysaccharide fine glassy particles is an effective way to preserve protein activity during the formulation process due to its resistance to organic solvents [5]. The present invention offers some improvements over previously reported methods [5,8] by avoiding charged polyelectrolytes, such as sodium alginate. The present invention also extends the usage of fine polysaccharide glassy particles for sustained-release of proteins from microspheres to various polymer-based devices, implants or injectable therapeutics.